Signal loss induced by superparamagnetic iron oxide particle in NMR spin‐echo images: The role of diffusion

MR contrast agents are used to improve the detectability of pathologic conditions. In this study we clarify the mechanism of action of a newly developed superparamagnetic agent (AMI‐25, Advanced Magnetics Inc., Cambridge, MA). Transverse relaxation rates were measured from segments of rat brain, heart, and liver using a CPMG pulse sequence. Relaxation rates varied with the time interval between the refocusing pulses in the sequence ( τ)The diffusion effect on signal intensity was quantitated from the variation of T₂with τ. We find that diffusion in the presence of microscopic field gradients reduces T₂ and that this effect differs in magnitude among tissues. Superparamagnetic iron oxide particles enhance this T₂reduction. The dependence of T₂upon τ suggests a model of restricted diffusion. Tissue image contrast induced by superparamagnetic iron oxide in spin‐echo NMR studies is the result of a combination of diffusion‐dependent and diffusion‐independent differences in transverse relaxation. © 1990 Academic Press, Inc.